The effective production of hydrocarbon reservoirs containing heavy oils or unconventional oils presents significant challenges. Extraction of these high viscosity hydrocarbons is difficult due to their relative immobility at reservoir temperature and pressure. These properties make it difficult to simply pump the unconventional oil out of the ground. Such hydrocarbons may be quite thick and have a consistency similar to that of peanut butter or heavy tars, making their extraction from reservoirs difficult.
Enhanced oil recovery or “EOR” processes employ thermal methods to improve the recovery of heavy oils from sub-surface reservoirs. The injection of steam into heavy oil bearing formations is a widely practiced enhanced oil recovery method. Typically, several metric tons of steam are required for each metric ton of oil recovered.
Traditionally, heavy oil recovery operations have utilized “once through” type steam generators also known herein as “Once Through Steam Generators (OTSGs). The steam or a steam-water mixture is injected via injection wells to fluidize the heavy oil. Different percentages of water and steam can be injected into the injection wells, depending on a variety of factors including the expected output of oil and the economics of injecting different water/steam mixtures.
Injected steam heats the oil in the reservoir, which reduces the viscosity of the oil and allows the oil to flow to a collection well. After the steam fully condenses and mixes with the oil, and is then produced, it is classified as “produced water.” The mixture of oil and produced water that flows to the production well is pumped to the surface. Oil is then separated from the produced water by conventional processes employed in conventional oil recovery operations.
There are many steam-based methods of enhanced oil recovery, but two important ones are cyclic steam stimulation or “CSS”, which uses vertical wells, and steam assisted gravity draining or “SAGD”, which uses a horizontal well pair. SAGD has come to be widely used in oil sands and there are also many variations on these methods.
For economic and environmental reasons, it is desirable to recycle produced water as much as possible. The produced water stream, after separation from the oil, is further de-oiled, and is treated for reuse. Most commonly, the water is sent to the “once-through” steam generators for creation of more steam for oil recovery operations. However, the reuse of water contributes significantly to build up of deposits in the OTSG and other steam handling equipment.
The tubes in OTSG's and downstream exchangers which cool steam and OTSG blowdown frequently foul with refractory deposits composed of both carbon and inorganic constituents. Pigging is a viable, but expensive, method for dealing with OTSG deposits. However, mechanical cleaning of some of the downstream exchangers is exceptionally difficult and expensive because of the high temperature, high pressure design features. Therefore, what is needed in the art is a means of chemically cleaning this equipment. At least one chemical vendor has proposed a chemical cleaning regimen using acidic and basic chemicals, but these chemicals pose corrosion, handling, and disposal risks.
Many attempts have been made to overcome these issues, but there still exists a need for a quick and cost effect means of cleaning steam and other SAGD equipment. Ideally, the treatment will not require expensive equipment or treatment systems, require large amounts of chemicals, increase energy consumption, or waste energy.